Diffusion STIR technique

Evaluation of the data requires time constants for both the heat transfer and diffusion under nonisothermal conditions. It turns out that the initial part of the response curve is mainly determined by the mass transport (transport diffusion), whereas the descending part is mainly governed by the heat transfer. The STIR technique avoids intrusion by the evolution of heat, in that it measures the transient temperature, and has a rapid response (time constant about 10 s). Results obtained for the diffusion of CH3OH into Na-X were essentially consistent with those derived from the ZLC and PFG NMR... 

Stirred suspensions of droplets have proven to be a popular approach for studying the kinetics of liquid-liquid reactions [54-57]. The basic principle is that one liquid phase takes the form of droplets in the other phase when two immiscible liquids are dispersed. The droplet size can be controlled by changing the agitator speed. For droplets with a diameter < 0.15 cm the inside of the drop is essentially stagnant [54], so that mass transfer to the inside surface of the droplet occurs only by diffusion. In many cases, this technique can lack the necessary control over both the interfacial area and the transport step for determination of fundamental interfacial processes [3], but is still of some value as it reproduces conditions in industrial reactors. 

Various techniques are available for determining the effective diffusivity of solute in gel (Itamunoala, 1988). One of the most reliable techniques is the thin-disk method which uses a diffusion cell with two compartments divided by a thin gel. Each compartment contains a well-stirred solution with different solute concentrations. Effective diffusivity can be calculated from the mass flux verses time measurement (Hannoun and Stephanopoulos, 1986). A few typical values of effective diffusivities are listed in Table 3.2. 

Apparent rate laws include both chemical kinetics and transport-controlled processes. One can ascertain rate laws and rate constants using the previous techniques. However, one does not need to prove that only elementary reactions are being studied (Skopp, 1986). Apparent rate laws indicate that diffusion or other microscopic transport phenomena affect the rate law (Fokin and Chistova, 1967). Soil structure, stirring, mixing, and flow rate all affect the kinetic behavior when apparent rate laws are operational. 

A comprehensive discussion of the most important model parameters covers phase equilibrium, chemical equilibrium, physical properties (e.g., diffusion coefficients and viscosities), hydrodynamic and mass transport properties, and reaction kinetics. The relevant calculation methods for these parameters are explained, and a determination technique for the reaction kinetics parameters is represented. The reaction kinetics of the monoethanolamine carbamate synthesis is obtained via measurements in a stirred-cell reactor. Furthermore, the importance of the reaction kinetics with regard to axial column profiles is demonstrated using a blend of aqueous MEA and MDEA as absorbent. 

Amperometric sensors — A class of electrochemical sensors based on amperometry. A - diffusion-limited current is measured which is proportional to the concentration of an electrochemically active analyte. Preferred technique for - biosensors with or without immobilized enzymes (biocatalytic sensors). The diffusion layer thickness must be kept constant, either by continuous stirring or by means of an external diffusion barrier. Alternatively, micro electrodes can be... 

Diffusivity Tests. Diffusivity tests were conducted for crushed catalysts (0.18- 0.26 mm) using the technique demonstrated by the previous workers (9). Changes in coronene concentration from 30 mg/cm in cyclohexane with time were measured in a tank stirred at 600 r.p.m. at an ambient temperature and pressure. Cyclohexane was dried with molecular sieve before use. The catalyst of about 200 to 300 mg, which was dried at 200 °C for three hours, was contained in the basket attached on the stirring shaft, and was soaked with 400 cm of the coronene solution in the tank. 2 cm of the solution was taken several times at a certain interval in five hours, and changes in the coronene concentration were measured with a UV spectroscopy. Coronene uptake was calculated from a change in its concentration in the tank. 

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